DD2 is 9 weeks old and has only STTN once which when the same day she got her vaccines. She usually sleeps from 9pm to 5am (she is usually up for 40 minutes) and then will sleep until 8am or 9am.
DD1 started STTN at 9 weeks so I'm assuming DD2 will probably start to do so soon since she only needs to sleep another 1 hour on the first stretch to be STTN. I count 9pm-6am as STTN.